1. Field of the Invention
This invention relates to a thermal head to be installed in a facsimile or a printer of various types, and more particularly to improvements in structure of a thermal head and in mounting structure for an integrated circuit device for driving a thermal head.
2. Prior Art
Conventionally, a thermal head of the line type or the serial type principally employs, as a driving system for heat generating resistor elements thereof, either a driving system of the direct driving type or that of the diode matrix type. In a thermal head employing either type of driving system, semiconductor element pellets such as ICs (integrated circuits) and diodes constituting a driving circuit device or the like are directly mounted on a thermal head substrate in order to allow miniaturization of the thermal head.
However, a thermal head of either type has many restrictions on reduction in size, reliability, price of products and so on, and improvement thereof is desired in these regards.
In particular, a conventional thermal head has a structure as shown in FIG. 17 wherein, in order to protect a semiconductor element 102 mounted on a substrate 101, the semiconductor element 102 is molded with encapsulating agent 103 and is further covered by an enclosure cover 104.
Further, in the thermal head of the type, it is necessary that the enclosure cover 104 can escape from a path of paper from the platen 105. In other words, the distance W.sub.1 from a heat generating resistor element to the semiconductor element 102 is restricted by a thickness t of the cover 104 and a dimension of a contour of the platen 105, and accordingly there is a limitation in miniaturization of the head. Besides, this limitation makes an obstruction to minimization of the substrate 101, and since a glazed ceramics material (Al.sub.2 O.sub.3) which is conventionally used in many cases as a material for substrates is expensive, it is desired for the head to be improved also from a point of view of a material cost.
Meanwhile, in the conventional thermal head, a structure is employed wherein a wear resisting layer is formed over the heat generating resistor element via an oxidation resisting layer in order to protect the heat generating resistor element from the platen 105.
However, since the oxidation resisting layer and the wear resisting layer are layered by a thin film forming technique such as sputtering, there is a restriction in assuring the life of the head against wear by employing a thick film for the wear resisting layer. If it is attempted to form a film, for example, of 10 microns thick for a wear resisting layer, much time is required for formation of the film, and cracks and some other defects may be caused by stress of the film upon formation of the film, resulting in reduction in reliability of the head.
Further, in order to assure the contacting characteristic of the head with the platen, the thickness of a film of electrodes by way of which the heat generating resistor element is fed is limited to 0.5 to 1.5 microns or so. Accordingly, a wire bonding operation is complicated, and there remains a problem in regard to the reliability of connection of the electrodes.
By the way, in the field of thermal recording, there is a tendency in recent years to reduce the size and improve the reliability of a thermal head, and such a conventional thermal head as described above cannot sufficiently meet those requests. Accordingly, it is desired for a thermal head to be improved in this regard.